According to the United Nations, there are over 100 million land mines currently deployed in more than 60 countries. The mines themselves range from large anti-tank mines to small anti-personnel mines and from all metal construction to primarily plastic or even wood. Triggering mechanisms range from direct pressure, to trip wires to magnetic sensors and fiber optics.
In addition, millions of bomblets were deployed as Cluster Bomb Units (CBUs) during wars and military actions. A significant number of these failed to explode and continue to threaten the populations indigenous to the original combat zones. Being largely constructed of metal, unexploded bomblets are readily detectable with existing hand-held metal detectors. However, current metal detectors have no way of discriminating an intact bomblet, which may be buried at depths up to 12 inches, from a bomblet fragment or other piece of shrapnel or metallic debris that is near the surface.
The US Army currently has a deployed mine detector called the AN/PSS-12. This is an inductive type detector that utilizes the creation of eddy currents in a metallic mine to alter the search coil impedance. This detector has served the Army well, but to be reliably detected, mines must be directly below the search head and must contain some metal. Other methods such as ground penetrating radar, infrared, and X-Ray have been investigated to solve the difficult problem of detecting low-metal and no-metal mines.
This invention relates to detection apparatus and methods which are capable of discriminating between mines, bomblets and other objects buried below the surface of the ground by detecting object depths, sizes, shapes, orientations and/or electrical properties. An inductive magnetometer is best suited to detecting and characterizing metallic objects; whereas, a capacitive dielectrometer is particularly effective in detecting and characterizing nonmetallic objects.
In the preferred magnetometer, a plurality of parallel, spaced linear conductor sets are disposed in proximity to the ground. An electromagnetic field is imposed in the ground with a dominant spatial wavelength through the conductor sets. A resulting electromagnetic response of the object in the ground to the imposed magnetic field is sensed. The method, in a preferred embodiment, also includes the step of translating electromagnetic response into estimates of one or more properties of the object based on a modeled response to the spatial wavelength.
In a preferred magnetometer embodiment, the dominant spatial wavelength has a length of at least 12 inches. The apparatus also has a rigid conductor element support structure adapted to be scanned across the ground.
In a preferred magnetometer, a primary winding has a series of parallel, spaced linear conductor sets driven by a current. The number of parallel conductors in the parallel conductor sets varies so as to shape the applied magnetic field. The applied field is periodic sinusoidal in a preferred embodiment.
The sensor in a preferred embodiment is an array of secondary windings. At least one of the secondary windings is located between parallel conductor sets of each pair of adjacent parallel conductor sets of the primary winding. The apparatus may have a second secondary array and primary winding which is perpendicular to the first set of parallel conductors of the first primary winding.
In a preferred embodiment of the dielectrometer apparatus, an excitation electrode carried on a sensor face is driven with a varying voltage, and a sensing electrode is carried by the sensor face. A guard electrode of the sensor face surrounds the sensing electrode and is at about the same voltage as the sensing electrode.
A shielding plane is located behind and spaced from the sensor face for blocking unwanted interference in one of the preferred embodiments of the dielectrometer apparatus. A guard plate is also interposed between the shielding plane and the guard electrode. A high-impedance buffer is connected to the sensing electrode to measure the magnitude and phase of the floating potential. The sensor face has an area of at least a square foot for mine detection but could be used in a smaller form for other applications, such as cure monitoring of thin coatings.
In one preferred embodiment of the dielectrometer apparatus, the sensing electrode has a plurality of elements in a column at different distances to the excitation electrode. In another preferred embodiment, the sensing electrode has a plurality of elements in a row wherein each element is equidistant to the excitation electrodes. The elements may be connected such that differences in measurements between adjacent elements can be used to detect small spatially abrupt changes in the dielectric properties, and to account for variations in stand-off distance from the sensor to the soil surface.